Well one thing I would suggest is, that you're a little underweight, relative to your size.

I would suggest you perhaps consider developing a little healthy muscle, as the Fat burning effects associated with muscle gain and improved metabolism, would be beneficial, plus if you're a bit "top heavy" and it's causing a bit of pain or discomfort, having a bit more muscle on the back as well as the front, would be beneficial for bearing the weight a little more easily, especially if you also lost a little Fat weight from the front too.

You could be better suited to another 15-20lbs of weight increase. It may sound like quite a bit, but trust me, you wouldn't look fat and it would be good for you. Although of course your weight may go up a little any before you're 21, as you're only 18 at the moment.

The best thing to do would be to join a gym if you can, or something like that.

This is the way you would probably start out.

3 days of Weights, 2-3 days of Cardio for about 20-30 minues per Cardio session.

Light weights initially to help you adjust, rising up to some heavier weight in the future.

As for the diet, you woudl be advised to increase calories, to permit exercise, I would aim for 2,100-2,200 a day. Sounds like a lot probably, but it will help, as an adult female needs around 1,800, but you would be better to have another 300-400 a day on top for exercise.

As far as that goes, you have options. Eating 6 times a day would be better, than three big meals, which might make you feel bloated and uncomfortable.